This study aims to identify the cell surface markers of rabbit lymphocytes that correlate with functional subpopulations and to use these markers to identify changes in the subpopulations when the rabbit is infected with retroviruses or is subject to autoimmune disease. Initial studies have concentrated upon the T -cell compartment and efforts are underway to identify the correlates of the human and murine markers for helper and cytolytic T -cells. Approaches to this problem have been through the use of monoclonal antibodies directed against cell surface antigens and be efforts directed toward molecular cloning of the genes for homologs of human CD4 and CD8. Antibodies have been used to monitor distribution of specific antigens in various cells and lymphoid organs. Further studies have used Western blot analyses determine the size of antigens recognized by the antibodies. The molecular approach to the identification of CD4 like molecules in the rabbit has used screening of a cDNA library prepared from rabbit lymphoid cells with synthetic oligonucleotide probes the sequence of which is deduced from common regions of the human, mouse, and rat CD4 homologs.